1. Field of the Invention.
This invention relates generally to methods and apparatus for measuring the formation of crystals in a flowing liquid, and more particularly, but not by way of limitation, to methods and apparatus for measuring the formation of wax crystals in cooled crude oil.
2. Description of the Prior Art.
The presence of waxes in crude oil poses significant problems, particularly in the transport of crude oil in a pipeline. Oil produced from offshore formations is recovered initially at a temperature in excess of the melting point of such waxes, so that initially at least the waxes are in a molten, liquid state. As this oil is transported through the pipeline from an offshore platform, however, the oil is cooled by heat transfer with the surrounding sea. Eventually the waxes present in the oil begin to solidify and form crystals. If for any reason the pumping of the oil through the pipeline is stopped, the wax crystals thus formed can gather and form a solid block to further movement of the oil through the pipeline. Substances known as pour point depressants have therefore been developed to inhibit the tendency of the waxes to solidify at the temperatures experienced in the pipeline.
A patent to Fischer, U.S. Pat. No. 2,668,470, discloses a method and apparatus for determining by electrooptical analysis the percentage of crystal solids formed from or in liquids at reduced temperatures, with particular application to analytical determination of paraffin wax in mineral oils and wax cakes upon elimination of turbidity. A filtered and polarized light beam from a rheostatted light source is passed through a crystallizing unit for a predetermined time interval, with the intensity of the light beam being gradually increased over the interval to record a mean optical activity for crystals formed in the crystallizing unit.
The crystallizing unit provides a crystal forming space in an otherwise insulated conduit containing a flow of the liquid, which space is formed by placing a series of uniformly spaced transparent plates between a pair of glass plates. Crystals are formed in and removed from the unit by ejecting at various times in the cycle of operation flows of cooling and heating air, respectively, against the unit. An insulated bypass is provided for diverting the flow of liquid away from the unit when the crystals are formed.
The mean optical activity over the predetermined interval of a system of unknown wax composition is compared with a set of curves produced for different wax compositions in the liquid. An indication of the percentage of crystal solids which are formed from or in the unknown system generally at reduced temperatures can be gained from such a comparison.
The device disclosed in the patent to Fischer does not, however, provide means for qualitatively monitoring the formation of crystals in a flowing liquid or for a comparative study of the rates of formation of such crystals in different systems and under a variety of conditions. Tests with various pour point depressants have indicated significant differences in the structure of wax crystals formed under identical conditions but for the presence of different pour point depressants. Tests have further demonstrated the impact of such differences on the tendency of such crystals to form effective blocks to flow in a pipeline at reduced temperatures.
An apparatus which provides means for qualitatively monitoring the formation of crystals in a flowing liquid, and/or for a comparative study of the rates of formation in different systems and under a variety of conditions, could be an especially effective tool in evaluating and suggesting means for combating the formation of an effective block of wax crystals in a pipeline.
It is therefore an object of this invention to provide for the qualitative monitoring of the formation of crystals in a flowing liquid.
It is another object of this invention to provide for such monitoring in a laboratory scale simulation of actual pipeline conditions.
It is a further object of this invention to provide an apparatus and method for evaluating the effectiveness of pour point depressants in inhibiting the formation of wax crystals in a pipeline.
It is a still further object of this invention to provide such an apparatus and method for evaluating the effectiveness of pour point depressants in terms of their effect on the structure and rate of formation of wax crystals in a pipeline.
Other objects and advantages will be apparent from a consideration of the following description, the appended claims, and the accompanying drawings.